The present invention relates generally to the formation of spin formable objects, such as vehicle wheels and other axial-symmetric parts, and more particularly to an improved method and apparatus for forming such a vehicle wheel or part while simultaneously heat treating the material.
Vehicle wheels may be conventional welded steel wheels or they may be formed as a one-piece wheel from a lightweight alloy, such as aluminum. These aluminum wheels are lighter than conventional welded steel wheels and reduce unsprung weight for better vehicle handling, as well as reduce weight for improved fuel efficiency. One-piece vehicle wheels of aluminum can be formed in a variety of different designs, allowing the customer to customize the look of his car or motorcycle. Automotive original equipment manufacturers (OEM) sales and after market sales in one-piece aluminum vehicle wheels are significant and the look of such wheels is considered desirable to a broad spectrum of the motoring public.
In a typical manufacturing operation, such as described in U.S. Pat. No. 4,528,734, a cast log or solid cylindrical length of aluminum is provided from a metals supplier and from which a several inch thick cylindrical billet is severed, which is then subjected to a series of hot forging operations to form the wheel center and a pair of rim flange legs. Thereafter, the forging is subjected to a trimming operation. The forged and trim wheel blank is then rough formed by a pair of spinning rollers which operate to axially elongate one of the rim flange legs and selectively vary the cross-sectional thickness of the flange legs. The rough form wheel is then subjected to a first solution heat treatment, after which final contouring and shaping is performed by additional spinning rollers. Finally, suitable machining cutters are used to face and machine to final tolerances the mounting surface portion of the wheel center, bead rim and other wheel parts prior to placing the wheel into service. Upon final contouring of the wheel, final machining of valve and bolt holes is done, followed by suitable appearance finishing and a final heat treating to provide a finished one-piece vehicle wheel. Heat treatment of aluminum is required to achieve the strength necessary for most lightweight aluminum parts. This requires the part, depending on the specific aluminum alloy, to be heated to 840 to 1075xc2x0 F., rapidly quenched in cold water and reheated to 240 to 400xc2x0 F. This thermal process strengthens the aluminum, but because of the severe thermal cycle and thermal shock, considerable distortion of the part occurs. Because the high temperature heat cycle is near the melting point of aluminum, the metal is soft and weak. The weight of the part at this high temperature causes the part to sag. This sagging distortion is increased considerably by the unequal thermal contraction of the wheel as it enters a cold water quench.
Several processes have been tried to minimize heat treatment distortion. These include:
1. Elaborate fixtures to hold the part rigid during the heating and quenching cycles.
2. Reducing the temperature gradient to lower the severity extremes of the quench. This is accomplished by chemical additions to the water bath to slow the cooling rate, raising the quench bath temperature to reduce the thermal difference or spray-quenching to reduce the cooling rate and thermal shock.
3. Mechanically stretching to straighten the part. This straightens the part and equalizes residual stress to lower distortion which must be machined out.
These procedures help minimize distortion from heat treatment, but in many cases, are not economical when compared to simply creating a thicker part to minimize distortion and then machining off the additional material.
Prior to the present invention, the inventor has produced a one-piece forged aluminum wheel from 6061-T6 alloy/temper which consists of starting with a cast aluminum billet log, 6 to 12 inches in diameter and 20 feet long. These logs are cut into a short length required to produce a billet. The cut piece is preheated to 800 to 1000xc2x0 F. and press-forged into the shape required for spinning. This forged shape is placed up on a spinning machine and spun at room temperature. This forms a wheel of rough dimensions with extra metal on all surfaces to allow final machining of the finished wheel. After spinning, the rough-formed wheel is placed into a heat treat furnace and solution treated at 960 to 1075xc2x0 F., followed by a rapid quench into water. This high temperature cycle and rapid quench causes wheel distortion. This requires extra metal on most wheel surfaces to guarantee the wheel can be machined to the required dimensions and tolerances.
Variations on the above process are disclosed in U.S. Pat. Nos. 4,936,129, 4,637,112, 4,532,786 and UK 2063722A. All of these patents disclose forming the wheel on a spinning machine wherein forming is done at room temperature using single-point contact radius rotors. Heat treatment is accomplished subsequent to spinning. Another technique is disclosed in U.S. Pat. Nos. 4,579,604 and 4,528,734 and involves spinning on a computer numerically controlled (CNC) machine. In this process, the wheel is spun with radius rollers on a mandrel after quenching and prior to a subsequent 350xc2x0 F. aging process. This forming operation removes distortion from the quenching and sizes the wheel to the mandrel, setting in the wheel the required dimensions and tolerances. The forming rollers are in point contact with the billet material and form the material at a rate that is much slower than that of a full contour roller used in the inventor""s following described rotary quenching process. The amount of metal reduction in this intermediate heat treatment process is limited to approximately a 50% reduction in wall thickness.
A rotary forging and quenching apparatus and method minimizes heat treatment distortion. This process requires forming during the normal heat treat quenching cycle. Rotary forging and quenching is a new process which combines the standard heat treatment process, rotary forging and CNC spin forming. Parts subjected to this process receive a first stage of heat treatment in a preheat furnace prior to the inventor""s process. These parts are rapidly transferred to the rotary forging and quenching machine, and formed and cooled during the quenching phase of a standard heat treatment process. The rotary forging and quenching machine is built similarly to a CNC spinning machine with critical modifications to accelerate the forming operation to coincide with the short time necessary for quenching. These changes require increased slide/roller forces, rapid slide/roller motion, full contour rollers, special high force part fixtures, rapid part fixture clamping, cooled machine bearings and CNC controlled part coolant. The rapid high force roller slide motion, rapid high force part clamping and full contact forming rollers are essential to accomplish the forming operation in the short time required to quench the metal. Rapid slide motion assures the slides are in position and that forming is accomplished during the cooling phase of the heat treatment process. Full contact rollers are necessary to form the final contact quickly. High force rollers and tailstock clamping are required to handle the forces required by full contact rollers.
Special part clamping fixtures are designed to clamp the part to minimize distortion, provide an inside rim contour support for the forming rollers and direct coolant to the rim center. Under the disclosed process, an aluminum part, such as of 6061 alloy aluminum, is preheated to the solution treatment temperature of 960-1075xc2x0 F. The part is rapidly transferred and clamped in a special rotary quench headstock fixture. This tooling is designed to promote sequential cooling of the part wherein the inside of the fixture directs coolant on the wheel center, cooling the center as the fixture is started rotating. Full contact rollers, such as one, two or multiple rollers, rapidly form the hot periphery of the part into the wheel rim contour. As the metal is formed to the rim contour, the cool fixture and coolant sprayed on the exterior rapidly cool the rim to complete the rotary forging and quenching process. Because this forming and quenching process is done on a mandrel fixture, it nearly eliminates the quench distortion that is exhibited by the standard heat treatment process. The center of the wheel is constrained during the cooling process and as a result, internal stresses within the wheel center are believed to improve the fatigue life of the wheel. Hot forming the rim portion with full contact rollers allows the rim material to be moved faster, further and with less roller force than could be accomplished at room temperature.
This rapid CNC-controlled sequential cooling and forming operation produces an effective heat treat quenching process, eliminates heat treat distortion and allows parts to be produced nearer to final dimensions while producing a beneficial stress condition in the finished part. The center is cooled first, before the outer, which places the center into a compressive stressed condition at room temperature. Wheels produced with a similar stress condition were reported by U.S. Pat. No. 4,767,473 to improve a wheel""s fatigue properties. The process results in lower material and machining cost and produces a longer lasting, high quality wheel.
One of the first applications for this process is the production of 6061-T6 aluminum wheels. However, this process is applicable to many round parts of aluminum alloy that require a solution treatment, quench and aging heat treatment process.